1. Field of the Invention
This invention relates to an X-ray projection exposure apparatus used in the manufacture of semiconductor integrated circuits.
2. Description of the Related Art
In solid-state devices, such as LSIs (large-scale integrated circuits) and the like, circuit patterns are becoming finer in order to increase the degree of integration and the operation speed thereof. In order to form such fine circuit patterns, reduction projection exposure apparatuses having vacuum-ultraviolet exposure light sources are widely used. The resolution of such a reduction projection exposure apparatus depends on the exposure wavelength .lambda. and the numerical aperture NA of the projection optical system. In conventional exposure apparatuses, an approach of increasing the numerical aperture NA is adopted in order to improve the resolution.
However, this approach is now close to the limit of the use because of the resulting reduction in the depth of focus and difficulty in the design and the manufacturing of the dioptric system. Accordingly, an attempt to shorten the exposure wavelength .lambda. is being made. For example, light used for exposure shifts from the g-line (.lambda.=435.8 nm) to the i-line (.lambda.=365 nm) of the mercury lamp, and further to KrF excimer lasers (.lambda.=258 nm).
Although the resolution of the apparatus is improved by shortening the exposure wavelength, there is a theoretical limit on the resolution from the wavelength of ultraviolet rays used for exposure. Accordingly, in the extended technique of conventional exposure apparatuses using light, it is difficult to obtain a resolution equal to or less than 0.1 um.
Against such a technical background, X-ray reduction projection exposure apparatuses using vacuum-ultraviolet rays or soft X-rays (these two kinds of rays are hereinafter termed "X-rays") as exposure light are attracting notice.